Patients of many different ailments often suffer from weaknesses or other inabilities to obtain a sufficient oxygen supply under their own power. In many cases, the patient cannot breath at all on their own and can only survive with the assistance of a ventilator. In other cases, the patient is able to breath on their own, but the energy expended by the patient in breathing could sap the patient of the energy needed to properly heal their ailments. In this case, the patient will benefit greatly from the use of a ventilator to assist the patient in their breathing by providing oxygen supplied from a ventilator.
The use of ventilators for patients having difficulty breathing is well-known. One type of ventilator that is commonly used is known as a positive-pressure ventilator. The positive-pressure ventilator forces the needed air, pure oxygen, or other gas mixture needed by the patient into the patient's lungs under an external pressure created by the ventilator. The ventilator forces the gases into the patient through an endo-tracheal tube, which may be placed through the patient's mouth or nose and directly into the patient's trachea. The ventilator can be adjusted to provide the patient with the proper mixture of gases, at the proper temperature, and a predetermined interval.
Such ventilators are configured to allow the patient to exhale only at predetermined times. Thus, when the machine determines that the patient should exhale, an exhalation valve is opened. The patient's lungs, acting against the reduced pressure in the system caused by the opening of the exhalation valve, force the gases out of the lungs and through the exhalation valve to the ambient atmosphere. After a predetermined time, the ventilator closes the exhalation valve, and the pressure build-up caused by the gas flowing from the ventilator fills the patient's lungs. Again, after the predetermined time described above, the exhalation valve is opened and the patient is permitted to exhale.
Such positive-pressure ventilators are useful for patients who cannot breath at all under their own power. Such ventilators may present difficulties, however, for use with patients who are capable of breathing or attempting to breathe under their own power, which is referred to herein as patient effort breathing. For instance, when a patient takes a spontaneous breath while under a positive-pressure ventilator, and the ventilator is not synchronized to the patient's breathing patterns, the ventilator may be forcing air into the patient's lungs while the patient is attempting to force air out of their lungs on their own. In these cases, the ventilator may actually cause unintended harm to the patient.
Ventilators have been developed which synchronize the ventilator's assistance to the patient's natural breathing pattern. These ventilators attempt to accurately determine the initiation of the patient effort breathing and, as quickly as possible, deliver a patient triggered breath, i.e., a breath delivered by a ventilator upon detection of the initiation of a patient effort breath. It is very important in these cases that the sensor sense the initiation of the patient effort breath as soon as possible and report this event via a trigger signal to the ventilator, so that the ventilator can properly provide the external pressure to assist the patient's breath, and not exert the external pressure while the patient exhales.
Numerous types of sensors are known in the art for sensing the initiation of a patient effort breath and triggering a signal to the ventilator. One type of sensor, shown in U.S. Pat. No. 5,513,631 to McWilliams, attaches to the external surface of the nose of the patient. The sensor is a pneumatic device that senses movement by compression of a constant-volume envelope. Movements from the patient's nose that occur as a result of the alae nasi reflex, which occur even prior to the diaphragm movement of the patient prior to a breath, are sensed by the pneumatic device and transmitted to the ventilator. While this device is adequate for its intended purpose, this device may not be entirely accurate under all conditions and in fact it may even be triggered by movements of the nose other than prior to a breath. It may also not provide a signal as soon as possible when the patient makes a patient effort breath.
U.S. Pat. No. 5,542,415 to Brody discloses a sensor for a ventilator for patient-assisted breathing. The sensor is taped to the abdomen of the patient and produces an output signal that is indicative of the movement of the diaphragm of the patient. The rate of change of the output signal with time is determined. When the rate of change with time exceeds a preselected value, a ventilation of the lungs of the patient by an external ventilator is initiated. While this device is adequate for its intended purpose, this device may not be entirely accurate under all conditions and in fact it may even be triggered by movements of the abdomen other than to take a breath.
Patient-assisted ventilators that sense the initiation of a patient effort breath by sensing changes in the flow rate of the gas being provided by the ventilator caused by the initiation of a patient effort breath are also known. For instance, U.S. Pat. Nos. 5,660,171 and 5,390,666 to Kimm et. al. disclose a system and method for flow triggering of pressure supported ventilation by comparison of inhalation and exhalation flow rates. The system provides a continuous flow of gas to the patient and provides additional gas to the patient when the system senses, by differences in flow rates, the patient's inhalation. The system utilizes a number of flow meters 16, 18, 32, 41, 42, and 43 to sense the rate of flow of gas through the system and utilizes two proportional solenoid valves 20 and 22 for controlling the flow of the air to and from the patient. While this device is adequate for its intended purpose, there is room to improve the device by eliminating the numerous flow meters thereby making the device less expensive to manufacture and more responsive to the patient's breathing. The flow meters may be prohibitively expensive, because there is a significant cost for each flow meter, and, in some cases, they must be changed for each patient. Further, ventilators often humidify the air and the patient's exhaled gases have 100% humidity. Thus there often is condensation around the flow meter, which may adversely effect the measurement of the flow of the gases.
Another method known for the detection of the initiation of a patient assist breath is the measurement of the change in pressure of the gases at the initation of a patient effort breath. There is known a pressure trigger device developed at the Massachusetts Institute of Technology that detects such changes in pressure. (See FIG. 1). The MIT ventilator trigger device employs a y-shaped connector having two separate check valves, an inhalation check valve, and an exhalation check valve, each located on a separate portion of the y-shaped connector. The configuration of the check valves provides a relatively small volume, approximately 10 cm.sup.3, separate from the rest of the relatively voluminous, approximately 600 cm.sup.3 of the patient circuit (the tubing to and from the patient and the ventilator). When the patient initiates a patient effort breath, the patient's lungs expand, thereby increasing the volume of the space between the patient's lungs and the check valves, and thereby lowering the pressure within this space. An opening in the wall of this chamber with a connection to a pressure transducer can measure this change in pressure and send a signal to the ventilator that the patient is making a patient assist breath.
The change in pressure caused by the patient assist breath is sufficient to overcome the cracking pressure of the inhalation check valve, thereby allowing the gases flowing from the ventilator to flow through the check valve through the connector, through the endo-tracheal tube, and into the patient's lungs. After a predetermined time, an exhalation valve is opened, which releases the pressure of the gas flowing to the patient, thereby closing the inhalation check valve. With the pressure to the patient's lungs reduced, the lungs compress to their resting state and the patient exhales. The increased pressure within the chamber of the connector against the exhalation check valve is sufficient to overcome the cracking pressure of the exhalation check valve, thereby opening the exhalation check valve and permitting the patient's exhalation gas to escape through the exhalation check valve and out the exhalation valve to the ambient atmosphere. Once the patient has exhaled, the pressure against the exhalation check valve is reduced and the exhalation check valve will close. The process can then begin again with the patient effort breath.
While the MIT ventilator trigger device is adequate for its intended purpose, it can be improved. For instance, the volume of the chamber of the MIT device, being about 10 cm.sup.3 is relatively large, thereby requiring rather large changes in pressure to cause the pressure sensor to react. In other words, the relatively large chamber reduces the sensitivity of the device to pressure changes caused by the initiation of the patient effort breath and may even cause the patient to exert additional force while attempting to breath before the sensor will sense the patient's breath. Thus, the strength of the patient effort breath may have to be relatively large to create a sufficient drop in pressure to be sensed by the pressure transducer.
Further, the relatively large volume creates the additional problem that the amount of exhaled gas from the patient may not be completely removed from the chamber when the exhalation check valve closes. Thus, there may be an unacceptable quality of gas exchange to the patient, with the possibility occurring that the patient may actually inhale up to 10 cm.sup.3 of exhaled gas. This may be a real concern with premature babies who have a tidal volume of between 8-10 cm.sup.3.
Accordingly, it will be appreciated from the foregoing that there is a definite need for an improved ventilator trigger device that has a greater sensitivity to changes in pressure caused by the initiation of a patient effort breath. The improved ventilator trigger device should be inexpensive to manufacture and use. It should be responsive to the breathing patterns of the patient and should not be triggered by events other than the initation of a patient effort breath. The improved ventilator trigger device should also have a minimum of moving parts and should be easy to clean and sterilize, or even be disposable.